In a typical data transmission system, a physical channel may be effectively modeled by utilizing filtration at the transmitter, channel filtration, an additive white Gaussian noise source, and receiver filtration. Certain methods have been developed for reliably transmitting digital information over linear distorting channels using linear modulation methods such as quadrature amplitude modulation (QAM) and phase shift keying (PSK). Powerful coded modulation methods have been utilized in a sufficiently high signal to noise ratio (SNR) environment to allow approaching a capacity of a Gaussian channel with intersymbol interference (ISI) if the system can attain ideal decision feedback equalizer (DFE) performance followed by Maximum Likelihood decoding. However realization of DFE in a coded system poses problems in obtaining reliable decision feedback, and hence, enhances severity of error propagation.
In the early 1970's Tomlinson modulo-precoding was introduced as a means to avoid error propagation in DFE for pulse amplitude modulation (PAM) systems by implementing a feedback filter of a DFE in a transmitter where a transmit symbol is utilized for feedback filtration. This method completely precompensates for postcursor ISI without increasing a transmitted power where moderate to high transmission rates are used. Modulo arithmetic is used to bound a dynamics range and to recover power loss implied by the filtration. More recently, the concept of precoding has been generalized to coded systems on partial response channels, onto higher dimensional signal sets such as QAM, and has been combined with trellis shaping to achieve substantial shape gain without reducing coding gain.
A precoding system may be realized using a noise predictive form of DFE together with a training procedure. Under moderate to high rate of operation, spectral properties of a precoded sequence remain statistically white, preserving characteristics of an input scrambled sequence to the precoding process.
In certain types of channel there is a need for a transmitted sequence to achieve certain spectral properties. For example, when a severe nonlinear distortion source is present in an output signal of a channel filtration, it may be desirable to include pre-emphasis filtration on the transmitted sequence such that a peak-to-average ratio of the channel output signal is reduced. However, typically pre-emphasis filtration as a component of linear equalization geared pre-emphasis spectrally shaping the transmit sequence imposes a power penalty on a transmit power-limited system, that penalty being independent of any previous coding, shaping and/or equalization method utilized in the transmission system. Higher than symbol-rate filtering is a convenient filtering rate for conventional transmit filters. However, at the present time, symbol-rate spectral shaping independent of the higher than symbol rate transmit filter is not available.
Thus, there is a need for a device and method that provide at least a substantially symbol-rate transmitter spectral shaping of signals representative of digital information that is independent of the higher than symbol-rate transmit filter.